Ultraviolet lamp systems are designed for coupling microwave energy to an electrodeless lamp, such as an ultraviolet plasma bulb, mounted within a microwave cavity or microwave chamber of the lamp system. In ultraviolet lamp heating and curing applications, one or more magnetrons are typically provided in the lamp system to couple microwave radiation to the plasma bulb within the microwave chamber. The magnetrons are coupled to the microwave chamber through waveguides that include output ports connected to an upper end of the chamber. When the plasma bulb is sufficiently excited by the microwave energy, it emits a spectrum of radiation having strong ultraviolet and infrared components through a bottom end of the microwave chamber toward a substrate to be illuminated. Typically, the ultraviolet lamp system also includes a metallic mesh screen mounted to the bottom end of the chamber that is transmissive to ultraviolet radiation but is opaque to the microwaves generated by the magnetrons. It will be appreciated that the terms "upper end" and "bottom end" are used herein to simplify description of the microwave chamber in connection with the orientation of the chamber as shown in the figures. Of course, the orientation of the microwave chamber may change depending on the particular ultraviolet lamp heating or curing application without altering the structure or function of the microwave chamber in any way.
The plasma bulb presents an approximate line source of a characteristic spectrum emitting radiation isotopically outward along its length. A portion of the emitted radiation moves directly from the plasma bulb toward the substrate without reflection. However, a large portion of the emitted radiation can only reach the substrate by undergoing one or more reflections. Ultraviolet lamp systems used in curing or heating of adhesives, sealants, inks or coatings, for example, include a reflector mounted within or that forms a part of the microwave chamber in which the plasma bulb is positioned. The reflector is an optical element operable to direct the reflected radiation in a predetermined pattern toward the substrate.
In most ultraviolet lamp systems, cold-light illumination of the substrate is desired in which the infrared component of the radiation emitted by the plasma bulb has been removed by absorption or reflection filters within the system. Infrared radiation interacts with the substrate to heat the irradiated surface. Therefore, the preferred reflector in such systems selectively reflects and transmits radiation as a function of wavelength. In particular, the reflector may be a coated optical element having a wavelength-selective coating that preferentially directs the ultraviolet radiation onto the substrate. Wavelength-selective coatings are typically multi-layered thin films of dielectric materials applied by vacuum deposition techniques familiar to one skilled in the art, such as physical vapor deposition or chemical vapor deposition. Radiation having wavelengths in the infrared range are transmitted through the reflector and absorbed by a surrounding structure to be dissipated thermally. Therefore, the extraneous infrared radiation is not substantially reflected by the reflector and does not illuminate the substrate.
The geometrical shape of the reflector determines the pattern of the radiation that illuminates the substrate. Reflectors are commonly shaped as a fragment of a conic section, such as a parabolic, elliptical, hyperbolic or circular section that is created when a solid cone is intersected by a plane. An elliptical reflector is commonly used to deliver a focused beam of ultraviolet radiation to the substrate, which is located at or near the focal plane of the reflector. A characteristic of an elliptical reflector is that a ray of radiation emitted from a source positioned at one focus will pass through the other focus after a single reflection. Thus, an elliptical reflector having a plasma bulb positioned axially along one focus will deliver a relatively focused line of radiation to a substrate positioned at or near the second focus.
Some applications for ultraviolet lamp systems, on the other hand, require a uniform irradiance or radiant flux density over a relatively large surface area of the substrate. A parabolic reflector having a source of radiation positioned at the focal point of the parabola would be ideal for such an application. Indirect radiation emitted from such a line source of the parabolic reflector will be reflected as a homocentric bundle of parallel rays toward a focal plane positioned at infinity. However, a parabolic reflector is relatively wide due to the mathematical description ascribed to the parabolic surface and, in most instances, a parabolic reflector will not physically fit within the available space in most microwave chambers.
In the past, designs of ultraviolet lamp systems have attempted to deliver a uniform irradiance by adjusting the optics of the reflector-plasma bulb-substrate configuration. One proposed solution to provide for a more compact reflector was to relocate the plasma bulb away from the focal point of an elliptical reflector. Although the ultraviolet radiation reflected by this configuration was at least partially defocused, the irradiance was not satisfactorily uniform across the surface area of the substrate. Moreover, the coupling between the incident microwave energy and the gas inside the plasma bulb is altered so that the radiation output from the bulb along its length is non-uniform. Another proposed solution was to reposition the elliptical reflector so that the substrate is no longer at or near the focal plane. However, repositioning the reflector reduced the available cooling for the plasma bulb and, concomitantly, reduced the useful lifetime of the bulb.
Thus, there is a need for a reflector that is configured to efficiently provide a uniform irradiance of ultraviolet radiation over a large area and that can do so without interfering with the optimum dimensions of the microwave chamber or adversely affecting the operating characteristics of the plasma bulb.